(1) Field of the Invention
The present invention relates to a developing device which performs development of a static latent image formed on a static latent image bearer with a toner as the coloring agent, and in particular, relates to a developing device using a developer as the toner composed of a mono component.
(2) Description of the Prior Art
An image forming apparatus using electrophotographic technology, such as a copier, a printer, has a developing device which, in order to visualize the static latent image formed on the surface of a photoreceptor as a latent image bearer, supplies a developer, such as toner or other coloring agents, to the photoreceptor and allows the toner to selectively adhere to the latent image.
In this developing device, the static latent image formed on the photoreceptor is developed and then the thus developed toner image is transferred to a sheet of paper as a printing material. On passing the transfer step, part of the toner which could not transfer to the sheet remains on the photoreceptor. This unnecessary leftover toner is removed from the photoreceptor in order to perform a next image forming operation. Therefore, a cleaning device for removal of the leftover toner from the photoreceptor is arranged downstream of the transfer station, and the unnecessary toner removed by this cleaning device is collected into a collecting portion in the cleaning device.
With a trend of image forming apparatuses having a developing device of the above type toward being compact, the space for arrangement of the processing devices for image formation around the photoreceptor needs to be reduced. So there is also a strong demand for a compact developing device.
In particular, the developing device for a dual component developer consisting of a toner and a magnetic carrier includes a magnetic brush type developing roller which conveys the dual component developer to the developing area facing the photoreceptor by the function of magnetism. In this developing device, the developer after development is collected into a developing hopper. Therefore, inorder to achieve a stabilized development, it is necessary to control the ratio of the toner contained in the developer, i.e., the toner concentration, so as to be constant, by re-supplying the toner being consumed.
Usually, for the developing device of the above type, i.e., using magnetic brush development, the carrier occupies a large volume in the developer, needing a large developing hopper for storing the developer, which leads to enlargement of the whole developing device. In addition, this type not only needs to control the toner concentration but also to have a multiple number of agitating means, for keeping the amount of electric charge on the toner in the developer uniform, which becomes the bottleneck for the downsizing of the developing device of this type.
In contrast, developing devices which perform development using a mono component developer, that is, only the toner as the developer with no carrier, have been proposed. In such a developing device using a mono component toner, there is no longer necessity to control the toner concentration, and the volume of the developer hopper can be markedly reduced because of the absence of carrier, making it possible to make the developing device compact. In addition, the mono component developing device is simple and excellent in maintenance. Specifically, the maintenance of the developer is simplified because the developer does not need to be replaced due to the carrier's degradation.
Control as to the toner also can be simplified because only the toner needs to be re-supplied while no toner concentration detection and hence its control will be needed. Particularly, in a developing device using a mono component toner, the only thing to be done is to re-supply the toner when it is needed.
For example, as shown in FIG. 3 opposed to a photoreceptor 1 as an image bearer is a developing device 4 for visualizing a static latent image formed on photoreceptor 1. In this developing device 4, a rotatable developing roller 41 is arranged so as to oppose the opening of a developer hopper 40 holding a toner 10, the mono component developer. This developing roller 41 is exposed in part to the opening of developer hopper 40 and arranged so as to be in contact with photoreceptor 1. This contact area will be the developing area.
Developing roller 41 supports the mono component toner on the surface thereof and conveys it to the developing area opposing photoreceptor 1. After development, the toner which has not been used for development is conveyed into developer hopper 40 and collected therein. The collected toner is once removed from the developing roller surface. For this purpose, a supplying roller 42 is provided in press contact with developing roller 41 so as to scrape the toner supported on the developing roller surface. This supplying roller 42 also functions to newly supply the toner to the developing roller surface.
The mono component toner is supplied by supplying roller 42 and adheres to the developing roller surface. In order to regulate the adhered amount, a regulating means 43 is arranged in pressing contact with the developing roller surface. The toner after having passed through regulating means 43 is regulated as to its amount and reaches the developing area where the toner opposes and is in contact with photoreceptor 1 as stated above. In this area, the toner selectively adheres to the static latent image formed on the photoreceptor surface, thus development is carried out.
Usually, a developing bias voltage (Va) is applied to developing roller 41 in order to perform beneficial development. This developing bias voltage is set at a level that allows the toner to adhere to the static latent image and no toner to adhere to the background area (the background with no image) on the photoreceptor.
By the above configuration, the mono component developer, or the toner is made to adhere to the developing roller and conveyed to the developing area so that the toner adheres to the static latent image on the photoreceptor while no toner will adhere to the background area other than the latent image, performing correct development.
The amount of toner adhering to developing roller 41 is limited by regulating means 43 as stated above. For this toner amount regulation, that is, in order to stabilize the amount of toner and stabilize the amount of charge on the toner, the supplying and tribo-charging of the toner by supplying roller 42 is of importance. Therefore, supplying roller 42 is pressed appropriately against developing roller 41, SO as to perform correct supplying and tribo-charging of the toner.
In this case, if the contact pressure against developing roller 41 varies, supplying and tribo-charging of toner 10 by supplying roller 42 fluctuates making correct development impossible. Particularly, supplying roller 42 is rotatably supported at both ends of its rotational shaft by means of bearings, so that the contact pressure of supplying roller 42 against developing roller 41 may differ between the middle area and the extreme end areas. Usually, the contact pressure tends to become low in the middle area. For this reason, toner supplying and toner's charge distribution become uneven due to variations in contact pressure along the direction of the rotational shaft, resulting in inability of stabilization of the toner concentration in the whole area.
Disclosed in Japanese Patent Application Laid-Open Sho 61 No.28978 is a supplying roller 42 for supplying developing roller 41 with the toner, which has a crowned configuration, having the maximum diameter at the center and the minimum diameter at both ends so as to realize approximately uniform contact pressure across the full length of the supplying roller 42 being in press contact with developing roller 41. With this configuration, supplying and charging of the toner can be made uniform along the axial direction, realizing a stabilized development.
As stated above, when supplying roller 42 is pressed against developing roller 41, formation of a crowned supplying roller 42 and abutment of it with an appropriate pressure stabilized supplying and uniform charge of the toner, thus enabling stabilized development. In this case, a more stable charge performance can be expected if the contact pressure between supplying roller 42 and developing roller 41 is set higher.
On the contrary, a stronger contact pressure imposes an increased load torque on the developing device, which means the necessity of greater driving torque for rotation of supplying roller 42. As a result, a driving motor. capable of producing a high driving torque is needed, leading to it being costly and enlargement of the developing device including the driving motor. Further, setting of a high contact pressure will promote wear of supplying roller 42, shortening the life of the developing device.
To avoid this, load torque needs to be reduced, that is, the contact pressure of supplying roller 42 against developing roller 41 may be weakened. However, this cannot be attained without no compromise in tribo-electrification performance and stability of toner supplying. That is, lowering of tribo-electrification performance and instability of toner supply causes image unevenness and white voids. To solve these problems, a crowned supplying roller 42 may and should be formed, but the fabrication is very difficult and hence the cost is greater. Further, the provision of a crowned supplying roller increases the driving torque.